1. Field of the Invention
The present invention relates to magnetic field sensors and, more particularly, to a fiber optic sensor which is highly sensitive to an applied magnetic field.
2. Description of the Prior Art
It is well known that fiber optic magnetic field sensors can be fabricated by cladding magnetostrictive material onto a length of optical fiber. When all or a portion of such a clad fiber is exposed to a magnetic field, oriented in a direction substantially parallel to the longitudinal axis of the fiber, the magnetostrictive cladding changes its length (e.g. elongates) which, in turn, changes (e.g. increases) the length of the optical fiber. As the field strength increases, the elongation correspondingly increases. This elongation varies the phase angle of any light propagating down the fiber and, more specifically, through any stretched portion of the fiber. Consequently, any increase in the elongation correspondingly increases the phase angle variation. Hence, by measuring phase angle variations by, for example, an interferometric technique, wherein any phase shift existing in a magnetically sensitive optical fiber "sensing" arm is compared to that occurring in an optical fiber "reference" arm, an accurate determination of the magnetic field strength can be readily and accurately obtained. This concept is more fully described in the following references: A. Yariv et al., "Proposal for Detection of Magnetic Fields Through Magnetostrictive Perturbation of Optical Fibers", Optics Letters, Vol. 5, No. 3, March 1980, pages 87-89; A. Dandridge et al., "Optical Fibre Magnetic Field Sensors," Electronics Letters, Vol. 16, No. 11, May 22, 1980, pages 408-409; G. Tangonan et al., "Current Sensing With Metal-Coated Multi-Mode Optical Fibers," Electronics Letters, Vol. 16, No. 25, Dec. 4, 1980, pages 958-959; H. Taylor et al., "Fiber Optic Sensors," Proceedings of the First European Conference on Integrated Optics 1981, pages 99-101; T. Giallorenzi et al., "Fiber Optic Sensors," Optics and Laser Technology, April 1981, pages 73-78; F. Trowbridge et al., "Metallic-Glass Fiber-Optic Phase Modulators," Optics Letters, Vol. 6, No. 12, December 1981, pages 636-638; and C. Davis, "An Introduction to Fiberoptic Sensors," Fiberoptic Technology, February 1982, pages 112-115.
However, the above-described fiber optic magnetic field sensor possesses serious drawbacks. Specifically, in order to produce a sensor with a high sensitivity, i.e., one that is able to detect a weak magnetic field, the magnetostrictive cladding must have a certain minimum thickness. Disadvantageously, this thickness cannot be readily obtained unless sophisticated and costly deposition techniques are used to clad magnetostrictive material to the optical fiber. In addition, the fiber must undergo extensive high temperature annealing during its manufacture in order to properly orient the grains of magnetostrictive material in the cladding so as to impart the desired bulk magnetic properties to the magnetostrictive cladding.